This invention relates to the recovery of P.sub.2 O.sub.5 from a gypstack in a wet process phosphoric acid operation. In particular, it relates to recycling the leachate from the gypstack back into the phosphoric acid process where the P.sub.2 O.sub.5 in the leachate is recovered.
Wet phosphoric acid processes for recovering P.sub.2 O.sub.5 from calcium phosphate rock involve dissolving the rock and precipitating calcium sulfate as the hemihydrate or dihydrate. In conventional wet process phosphoric acid production, the byproduct calcium sulfate, either in the dihydrate and/or hemihydrate form, is stacked on open ground forming a gypstack. The solid calcium sulfate is pumped as a slurry in pondwater to the top of the stack, and the solids settle out on top of the stack. Most of the pondwater, now free of solids, overflows the stack and runs back to the main pond system. Part of the pondwater seeps through the stack and the resulting leachate commonly flows into a canal surrounding the stack.
Varying P.sub.2 O.sub.5 losses occur in the phosphoric acid process, due to calcium phosphate that is not dissolved, soluble P.sub.2 O.sub.5 that is not washed out of the cake, and to P.sub.2 O.sub.5 that is bound in the lattice structure of the calcium sulfate waste product. Higher P.sub.2 O.sub.5 losses occur in the hemihydrate process when compared to an efficiently run dihydrate process.